The invention relates to a method and a device for controlling the frequency of a travelling wave motor of the type having a disk-shaped stator with a piezoring fastened to the stator and a disk-shaped rotor which is pressed to contact the piezoring via a frictional layer, with the piezoring carrying two groups of mutually alternating piezoelectric zones which are excited by respective alternating voltages phase-shifted by 90.degree. in relation to one another, and wherein an actual value proportional to the actual frequency of the motor is obtained from the amplitude of an alternating voltage (sense voltage) tapped from the stator of the traveling wave motor, is compared with a desired value proportional to the desired frequency, and the resulting deviation value is used to correct the frequency of the excitation alternating voltage of the motor after being subjected to a suitable control algorithm.
Travelling wave motors, also referred to as ultrasonic motors, are known, and their construction, their function, and their preferred possible applications are described, for example, in the following literature:
Schadebrod and Salomon, "Der Piezo-Wanderwellenmotor--Antriebselement in der Aktorik, Neues Wirkungsprinzip," (The Piezo Traveling Wave Motor--Driving Element in Actuation; A New Performance Principle), Industrie-Anzeiger 6/1991, Pages 32-34;
Schadebrod and Salomon, "The Piezo Traveling Wave Motor--A New Element in Actuation," Control Engineering/May 1990, Pages 10-18.
Yukihiko Ise, "Traveling Wave Ultrasonic Motors Offer High Conversion Efficiency, JEE, June 1986, Pages 66-70.
In order to control the traveling wave motor, a zone of the piezoring in the stator is contacted a sensor output. This zone gives off an alternating voltage, the so-called sense voltage or "sense," which is used to control the motor The object of the frequency-control method for the known traveling wave motor of the type mentioned above is to keep the operating point of the traveling wave motor within the resonance range of its characteristic curve, since it affords maximal performance in this operating range.
In a known method of this type, the desired value of the frequency of the excitation voltage is changed manually by means of a desired value transmitter until the user realizes on the basis of the change in the speed and by the sounds emitted by the traveling wave motor that the latter is within the resonance range. The desired value is then left unchanged during operation. This adjustment process must be performed for each traveling wave motor due to the spread between units and repeated each time the traveling wave motor is used again, since the operating point is displaced due to internal processes in the traveling wave motor while the traveling wave motor is at rest, although the desired value was not changed. Nor does the optimal operating point during operation lie at a constant frequency, which is additionally subjected to the spread between units, but instead is subjected to the drifting of a plurality of parameters so that the known frequency control using a constant predetermined desired value does not result in a stable operating point within the maximum performance of the traveling wave motor.